How do I use Comparator to define a custom sort order

Question

I want to develop a sorting demo for car list  I am using data table to display car list  Now actually I want to sort the list by car color  Here it is not sort by alphabetic order  I want to use my custom sorting order like Red car come first  then Blue  etc   For that I try to use Java Comparator and Comparable but it allows to sort in alphabetic order only   So  can any one guide me the way to implement the technique to use so that the sorting becomes faster    class Car implements Comparable lt Car gt        private String name      private String color       public Car String name  String color           this name   name          this color   color               Implement the natural order for this class     public int compareTo Car c            return name compareTo c name              static class ColorComparator implements Comparator lt Car gt            public int compare Car c1  Car c2                String a1   c1 color              String a2   c2 color              return a1 compareTo a2                        public static void main String   args            List lt Car gt  carList   new ArrayList lt  gt             List lt String gt  sortOrder   new ArrayList lt  gt              carList add new Car  Ford   Silver             carList add new Car  Tes   Blue             carList add new Car  Honda   Magenta              sortOrder add  Silver            sortOrder add  Magenta            sortOrder add  Blue                Now here I am confuse how to implement my custom sort

User · Answer

Comparator in line      List lt Object gt  objList   findObj name   Collections sort objList  new Comparator lt Object gt           Override     public int compare Object a1  Object a2            return a1 getType   compareToIgnoreCase a2 getType

User · Answer

Define one Enum Type as   public enum Colors        BLUE  SILVER  MAGENTA  RED     Change data type of color from String to Colors Change return type and argument type of getter and setter method of color to Colors  Define comparator type as follows  static class ColorComparator implements Comparator lt CarSort gt        public int compare CarSort c1  CarSort c2                return c1 getColor   compareTo c2 getColor               after adding elements to List  call sort method of Collection by passing list and comparator objects as arguments  i e  Collections sort carList  new ColorComparator      then print using ListIterator   full class implementation is as follows   package test   import java util ArrayList  import java util Collections  import java util Comparator  import java util List      import java util ListIterator   public class CarSort implements Comparable lt CarSort gt        String name      Colors color       public CarSort String name  Colors color           this name   name          this color   color              public String getName             return name            public void setName String name            this name   name            public Colors getColor             return color            public void setColor Colors color            this color   color               Implement the natural order for this class     public int compareTo CarSort c                return getName   compareTo c getName                static class ColorComparator implements Comparator lt CarSort gt                public int compare CarSort c1  CarSort c2                        return c1 getColor   compareTo c2 getColor                          public enum Colors            BLUE  SILVER  MAGENTA  RED             public static void main String   args                  List lt CarSort gt  carList   new ArrayList lt CarSort gt              List lt String gt  sortOrder   new ArrayList lt String gt               carList add new CarSort  Ford Figo  Colors SILVER             carList add new CarSort  Santro  Colors BLUE             carList add new CarSort  Honda Jazz  Colors MAGENTA             carList add new CarSort  Indigo V2  Colors RED             Collections sort carList  new ColorComparator               ListIterator lt CarSort gt  itr carList listIterator             while  itr hasNext                  CarSort carSort    CarSort  itr next                System out println  Car colors    carSort getColor

User · Answer

I think this can be done as follows   class ColorComparator implements Comparator lt CarSort gt        private List lt String gt  sortOrder      public ColorComparator  List lt String gt  sortOrder           this sortOrder   sortOrder             public int compare CarSort c1  CarSort c2                String a1   c1 getColor            String a2   c2 getColor            return sortOrder indexOf a1  - sortOrder indexOf a2               For sorting use this    Collections sort carList  new ColorComparator sortOrder

User · Answer

In Java 8 you can do something like this   You first need an Enum   public enum Color       BLUE  YELLOW  RED     Car class   public class Car        Color color                 public Color getColor             return color             public void setColor Color color            this color   color            And then  using your car list  you can simply do   Collections sort carList  Comparator comparing CarSort  getColor

User · Answer

How about this   List lt String gt  definedOrder      define your custom order     Arrays asList  Red    Green    Magenta    Silver     Comparator lt Car gt  comparator   new Comparator lt Car gt           Override     public int compare final Car o1  final Car o2              let your comparator look up your car s color in the custom order         return Integer valueOf              definedOrder indexOf o1 getColor                  compareTo                  Integer valueOf                      definedOrder indexOf o2 getColor                  In principle  I agree that using an enum is an even better approach  but this version is more flexible as it lets you define different sort orders   Update  Guava has this functionality baked into its Ordering class   List lt String gt  colorOrder   ImmutableList of  red   green   blue   yellow    final Ordering lt String gt  colorOrdering   Ordering explicit colorOrder   Comparator lt Car gt  comp   new Comparator lt Car gt           Override     public int compare Car o1  Car o2            return colorOrdering compare o1 getColor   o2 getColor                 This version is a bit less verbose     Update again  Java 8 makes the Comparator even less verbose   Comparator lt Car gt  carComparator   Comparator comparing          c - gt  definedOrder indexOf c getColor

User · Answer

I recommend you create an enum for your car colours instead of using Strings and the natural ordering of the enum will be the order in which you declare the constants   public enum PaintColors       SILVER  BLUE  MAGENTA  RED     and   static class ColorComparator implements Comparator lt CarSort gt          public int compare CarSort c1  CarSort c2                  return c1 getColor   compareTo c2 getColor                 You change the String to PaintColor and then in main your car list becomes   carList add new CarSort  Ford Figo  PaintColor SILVER          Collections sort carList  new ColorComparator

User · Answer

I will do something like this   List lt String gt  order   List of  Red    Green    Magenta    Silver     Comparator comparing Car  getColor    Comparator comparingInt c - gt  order indexOf c      All credits go to  Sean Patrick Floyd

User · Answer

I had to do something similar to Sean and ilalex s answer  But I had too many options to explicitly define the sort order for and only needed to float certain entries to the front of the list     in the specified  non-natural  order  Hopefully this is helpful to someone else   public class CarComparator implements Comparator lt Car gt           sort these items in this order to the front of the list      private static List lt String gt  ORDER   Arrays asList  dd    aa    cc    bb         public int compare final Car o1  final Car o2            int result   0          int o1Index   ORDER indexOf o1 getName             int o2Index   ORDER indexOf o2 getName               if neither are found in the order list  then do natural sort           if only one is found in the order list  float it above the other           if both are found in the order list  then do the index compare         if  o1Index  lt  0  amp  amp  o2Index  lt  0  result   o1 getName   compareTo o2 getName             else if  o1Index  lt  0  result   1          else if  o2Index  lt  0  result   -1          else result   o1Index - o2Index          return result           Testing output  dd aa aa cc bb bb bb a aaa ac ac ba bd ca cb cb cd da db dc zz

User · Answer

Using just simple loops   public static void compareSortOrder  List lt String gt  sortOrder  List lt String gt  listToCompare           int currentSortingLevel   0          for  int i 0  i lt listToCompare size    i                 System out println  Item from list      listToCompare get i                System out println  Sorting level      sortOrder get currentSortingLevel                if  listToCompare get i  equals sortOrder get currentSortingLevel                    else                   try                      while   listToCompare get i  equals sortOrder get currentSortingLevel                            currentSortingLevel                        System out println  Changing sorting level to next value      sortOrder get currentSortingLevel                      catch  ArrayIndexOutOfBoundsException e                                                       And sort order in List  public static List lt String gt  ALARMS LIST   Arrays asList               CRITICAL                MAJOR                MINOR                WARNING                GOOD                N A

[java] How do I use Comparator to define a custom sort order?

Examples related to java

Examples related to sorting

Examples related to enums